Generally, there has been known an electrostatic coating apparatus which is constructed of, for example, an atomizer consisting of an air motor and a rotary atomizing head, a housing member formed of an electrically insulating material and adapted to hold the air motor of the atomizer in position, a tubular cover member arranged to cover outer surfaces of the housing member, and a high voltage generator adapted to electrify atomized paint particles with a negative high voltage electrostatic charge as the paint particles are sprayed forward from the rotary atomizing head of the atomizer by using external electrode assembly (e.g., Japanese Patent Laid-Open No. 2001-113207).
In electrostatic coating apparatuses of this sort, an electrostatic field is formed by lines of electric force between an external electrode, to which a negative high voltage is applied, and a rotary atomizing head which is held at the earth potential, and between the external electrode and a work piece. Besides, a negative ionization zone is formed in the vicinity of a fore distal end of the external electrode assembly.
If, in this state, paint is sprayed by a rotary atomizing head which is put in high speed rotation, sprayed paint particles are electrified by application of a negative high voltage during travel through the ionization zone to become negatively charged paint particles. As a result, the charged paint particles are urged to fly toward and deposit on surfaces of a work piece which is connected to the earth.
In the case of the electrostatic coating apparatus of above-mentioned Japanese Patent Laid-Open No. 2001-113207, outer surfaces of the cover member are electrified to negative polarity by discharged negative ions. Therefore, repulsion takes place between the cover member and paint particles which are electrified to the same polarity, preventing the paint particles from depositing on outer surfaces of the cover member. In addition, the cover member which is formed of an electrically insulating material can prevent high voltage electrostatic charges on its outer surfaces from leaking to the side of the earth potential.
However, actually, as an electrostatic coating operation is continued, paint particles start to gradually deposit on outer surfaces of the cover member and remain there as a paint deposit. This paint deposit is problematic in that it gives rise to degradations in insulating performance of the outer surface of the cover member. Degradations in insulating performance of the cover member are reflected by paint deposition progressing at an abruptly increasing rate. Therefore, it is often the case with conventional electrostatic coating apparatuses that coating operations are interrupted frequently for removal of paint deposits.
Further, in the electrostatic coating apparatus according to the above-mentioned Japanese Patent Laid-Open No. 2001-113207, paint deposition is prevented by coating a water repellent paint on outer surfaces of a cover member. However, this electrostatic coating apparatus requires to coat water repellent paint periodically because outer surfaces of the machine need to be washed repeatedly after finishing coating operations and as a result the thickness of the water repellent coating becomes thinner and thinner. In addition, because of instability in quality, the use of the water repellent paint involves such problems as low yield of products and costly coating operations.